ceph: check if file lock exists before sending unlock request

When a process exits, kernel closes its files. locks_remove_file()
is called to remove file locks on these files. locks_remove_file()
tries unlocking files even there is no file lock.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>